1. Field of the Invention
This invention relates to a process wherein the rapidly moved surface of an aqueous dispersion on the basis of polyvinyl chloride is contacted in a plurality of successive chambers, in each chamber with a secluded intensely mixed gas atmosphere, the gas being transported from one chamber to the next one in one direction and the dispersion being transported in the opposite direction. The invention also relates to a device suitable to carry out the said process and comprising a horizontal cylindrical vessel divided into chambers by partition walls in vertical position relative to the cylinder axis, an inlet and an outlet for the dispersion, an inlet and an outlet for the gas and at least one stirrer in each chamber.
Plastics dispersions as obtained by homopolymerization, copolymerization or graft polymerization of vinyl chloride (VC) in aqueous emulsion or suspension or by subsequent dispersion of the finished polymer often contain volatile constituents, especially unreacted monomers, the removal of which is desirable for various reasons.
These volatile constituents may detrimentally effect the processing properties of the dispersions or of the solid powders obtained therefrom, as well as the properties of the products made therefrom. For example, bubbles may be formed, the material may have an unpleasant odor, the mechanical properties may be deteriorated and the thermostability reduced. It is especially important to remove as far as possible volatile constituents which are physiologically noxious or unhealthy. In this case, the volatile constituents should be removed without injurious emission into the environment.
Physiologically hazardous, readily volatile constituents are often the unreacted monomers, especially vinyl chloride (VC), the content of which in the polymer should not exceed 10 ppm as a result of more recent discussions about possible official regulations.
2. Description of the Prior Art
Various processes are known to remove volatile constituents from plastics dispersions by treating them with gases, especially with steam.
It has been proposed to free the polymer dispersion from monomers by passing through steam or inert gases. For this purpose considerable amounts of steam or gas are required which cause troubles, above all with polymer dispersions tending to foam. The addition of anti-foaming agents renders the process more expensive, it is not always effective and may detrimentally affect the properties of the dispersion. To avoid the difficulties, it has been proposed to destroy the foam formed during the passage of steam or inert gases by rapidly reducing the pressure at steam velocities of more than 100 m/sec, to separate the steam from the broken foam and to recycle the latter into the boiling dispersion. The process is carried out continuously and requires a high amount of steam.
In another known process the dispersion is mixed in a tube with steam or hot inert gases and the formed three-phase mixture is separated shortly thereafter into dispersion and gaseous phase. It has also been proposed to atomize polymer dispersions in steam flowing at a high speed, to pass the mixture obtained between heated plates into a release zone where the dispersion is separated from the gaseous phase in a cyclone. A further known process is carried out in analogous manner, the aqueous dispersion and the stripping gas are injected into a contacting tube substantially without contraction, in which tube the vaporous mixture of dispersion and stripping gas travels at a high speed. The mixture is then passed through a nozzle into a further contacting tube without contraction and finally discharged into a separating chamber at reduced pressure. The three latter processes are carried out in continuous manner but all require high amounts of steam. In spite of the high velocity of flow the dispersion may be superheated, coagulations may occur and the polymer may cake on the apparatus walls. Moreover, monomers included in the polymer core, which require a certain time of diffusion, are removed very incompletely only.
In another known process the dispersion is conducted in a column in countercurrent flow to a steam current. In this case, too, foaming may occur with a high throughput of dispersions of low surface tension and deposits of solids may easily clog the apparatus.
It has also been proposed to spray the dispersion into a steam atmosphere in such a manner that the sprayed particles fall through the steam owing to gravity. For this purpose a large and relatively expensive vessel is required.
In a further known process the plastics dispersion is treated in parallel or countercurrent flow in a trickling absorber with an inert gas current at a temperature of from 40.degree. to 100.degree. C. For a substantial removal of volatile substances down to a few ppm (calculated on the dispersion) the throughput must be kept relatively small or a large apparatus is required. The tubes of the absorber may easily narrow or clog by increasing deposit formation, especially with dispersions with settling tendency of the dispersed matter, so that capacity and heat transition are reduced. With the high amounts of gas required the recovery of volatile constituents from the inert gas is difficult and expensive. On the other hand, with physiologically objectionable volatile substances an emission of the inert gas containing same is to be avoided for reasons of environmental protection.
Finally, a device has been proposed to expel solvent residues from polymer dispersions by steam distillation consisting of a horizontal vessel provided with vertical weirs. The dispersion is supplied at one end of the vessel and travels from one compartment to the other by overflowing the weirs. Each compartment of the vessel is equipped with a multistage-impulse-countercurrent stirrer and has a steam inlet at the bottom. The steam flows through the dispersion and is discharged, together with the expelled solvent, from a common gas space in the upper section of the vessel. After having passed all weirs in the vessel the dispersion is discharged.
This device is little suitable for removing volatile constituents from plastics dispersions with tendency to settle of the dispersed particles with possible formation of a layer on the bottom which cakes in permanent contact with the steam blown in and may clog the inlet opening. When steam is blown through dispersions tending to foam, difficulties may arise by the formation of foam. In order to remove the volatile constituents as completely as possible, long residence times and a relatively high consumption of steam are required. The recovery of such constituents, which must not be emitted into the atmosphere, is rendered difficult owing to the high amount of carrier gas. In the case of dispersions, which are sensitive to elevated temperatures, prolonged residence times may cause damages. The liquid level in the vessel is fixed by the weirs and thus the residence time with a constant volume flowing through. The multistage-impuls-countercurrent stirrer is suitable to produce a laminar flow and developed for low circumferential speeds. In the turbulent range high efficiencies of revolution cannot be achieved.